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Glaze Chemistry

Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.

Key phrases linking here: glaze chemistry - Learn more

Details

Glaze chemistry is learning what each oxide does in a fired glaze and the relative advantages and disadvantages of each material supplying it. The chemistry of a glaze is expressed in a manner similar to its recipe, except that the items are oxides and the amounts can be by weight (an analysis) or numbers of molecules (a formula).

There are close links between the way glazes fire in a kiln and their chemistry. Physical properties like color, hardness, melting temperature, thermal expansion, leachability, etc. are all direct products of the oxides present in their chemistry. The relationship between chemistry and fired properties of glazes is a matter of much study. Companies who understand specifics within certain temperature and process systems have often leveraged this knowledge to develop important products. Knowing about chemistry is also important to being able to substitute materials in recipes while maintaining their chemistries.

Understanding the relationship between the absolute and relative amounts of the common oxides appearing in a typical glaze formula to the physical presence of a fired glaze itself is the key to control. An education in glaze chemistry usually starts with a study of each of the dozen or so oxides that a fired glaze is conceptually structured from and what properties each contributes to the fired glaze (and their interactions). Oxides are divided into three main categories (which flux, stabilize and build glass), oxides within each contribute more specifically to properties of the fired glass. A study of what materials contribute what oxides usually follows. Finally people learn how to use a computer program to study existing glazes and explain their behavior and start adjusting them to alter properties or fix problems. Understanding this is well within the reach of anyone and is actually much simpler than trying to grasp the relationship between the recipe of a glaze and its firing behavior. Software, like an account at insight-live.com, automates tracking of material analyses and calculating the composite formula of a mix of them.

Glaze chemistry is not an absolute science. Many other factors interplay with the chemistry (e.g. physical presence of materials, process) to affect how we can rationalize the degree to which the chemistry is responsible for fired properties we seek. This imprecision makes glaze chemistry a relative science. There are no absolute rules about what ratios or amounts should be (although technicians do learn general ranges with experience in testing and assessing existing recipes). Expansion calculations are an example. They are relative within a system (material or oxide). For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of minor variations, the calculated expansions will give you a fairly accurate indication of which variations have higher and lower expansions and to what degree. But if you make a major change to the oxide balance (perhaps moving toward a high MgO from a high KNaO content) while still using the same materials, the calculated thermal expansion difference between the two will often not accurately reflect the actual measured differences. Even more significantly, if lithium carbonate, for example, is introduced (or a boron frit, or zinc, for example) it is now a different system and different factors affecting the proximity of calculated and actual thermal expansion come into play.

Glaze chemistry cannot be taken in isolation from the character of the minerals and materials that supply the oxides. The manner and temperatures at which they decompose to liberate their oxides is determined by many factors. For example, a mineral and frit could have close to the same chemistry but they melt in completely different ways (because the former is crystalline and the latter is a glass). Calcined alumina appears to be just Al2O3 on paper, but incredibly high temperatures are needed to convince it to release its oxide to a glaze melt. So, while in theory, chemistry considers all materials on an equal footing as suppliers of oxides, in practice materials themselves have unique melting behaviours. A glaze will melt much better if it sources MgO from a frit instead of dolomite or talc. Feldspar far more readily releases SiO2 to the melt than does raw quartz. In fact, quartz particles literally need to dissolve in a melt of other things, be taken into solution in it, normal kiln temperatures will not melt them. Other materials must interact, they do not melt on their own (e.g. calcium carbonate and dolomite are refractory enough to be used for making kiln bricks, but when combined in the right way with others, will actively melt).

There are other factors that affect the accuracy of thermal expansion prediction:
-Some oxides, like Li2O do not impose their expansions in a linear fashion or in accordance with their proportion.
-The degree to which a glaze is completely melted determines the degree to which its expansion calculation is valid.
-Crystallization: A melt that freezes to a crystalline solid has a very different thermal expansion than when it freezes to a glass.
-Non melting particles: Zircon imposes its expansion as a non-participant in the glass structure, this is very different than if it were to melt and participate in the glass chemistry. Alumina (calcined vs hydrate) is a similar story.
-Oxides interact: The expansion contribution of one may be different depending on which other oxides are present.

As noted, in glaze chemistry we apply our knowledge of how oxides affect fired glazes to propose changes that have a high likelihood of moving a glaze property (e.g. melting temperature, thermal expansion, color compatibility) in a certain direction. We then make fired tests and compare the magnitude by which physical changes vary from the original (changes we can see and measure). Finally we propose additional changes or testing directions based on what has been learned. An important skill that is learned over time is the ability to separate the chemistry from the effects of material mineralogy and physics and process and firing, how to determine the degree to which each is affecting the test results.

Without glaze chemistry you cannot properly answer:
• Why is my glaze leaching? Is it likely to leach?
• Why is this glaze so touchy and difficult to keep consistent?
• How do I fix crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering?
• How can I change the firing temperature? The gloss? Mattness?
• Why doesn’t this colorant work in this base glaze?
• How do I substitute materials that are expensive, problematic, unavailable? One frit for another?
• How can I best utilize raw materials that I have easy and inexpensive access to? Use them in the highest possible percentages?
• Can I predict what possible weaknesses my glazes will have?
• How can I create and optimize base glazes that I can opacify, crystallize or variegate?

Chemistry in Clay Bodies

As noted, glaze chemistry is a relative science and changes to the chemistry of a glaze are interpreted as to what it will do in a relative sense (e.g. melt lower, thermally expand more, be more or less matte, etc). The chemistry of bodies is not normally considered indicative of fired properties (for example, one cannot calculate the thermal expansion of a clay body). Thermal expansion calculations assume a glass where all oxides can impose their proportionate expansion on the whole. This does not work for crystalline solids. Clay bodies do not melt like glazes, the oxides do not form a homogeneous glass, they undergo complex crystallization while cooling in the kiln. A glass and crystal of the same chemistry usually have wildly different physical properties. Variations in particle size distribution, particle mineralogy and shape, firing speed, atmosphere and duration of firing all affect the progressive stages of decomposition and play out of interactions that break and build molecular bonds; these variations are evident in the fired product and all beyond the scope of the chemistry. Consider SiO2 oxide content: It may be equal in two bodies, but one may have most of the SiO2 in quartz grains and the other might have it as a molecular component of feldspar and kaolin. These will, of course, have vastly different thermal expansions.

Related Information

Trying to avoid knowing anything about glaze chemistry? Be ready for drama!

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A periodic table of oxide with elements important to ceramics highlighted

Maybe you don't think it is necessary to know anything about glaze chemistry to be a potter. Or a technician at a production facility. This thinking depends on how much mystery you mind tolerating. Because the reason for many of the problems you will encounter with glazes relates fundamentally to their chemistry. Perhaps you have other "social" actors in your sphere who also specialize in the "know as little technical stuff as possible" mindset. Who treat glazes like acrylic paint that comes in tubes - it is just color! From these people, you will get buying advice on expensive jars of tacky-looking "goop" that you have to laboriously paint on in layers. Or it will mean you'll be more likely to get trapped on the recipe treadmill, addicted to the traffic of recipes that never seem to work.

Glaze chemistry works best when changing material amounts, not material types

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Two glazes, same chemistry, different materials. The glaze on the left is sourcing CaO from wollastonite, the one on the right from calcium carbonate. Thus the oxide chemistry of the two is the same but the recipe of materials sourcing that chemistry is different. The difference in this GLFL test for melt flow is an expression of how choosing different mineral sources to source an oxide can produce melting patterns that go outside what the chemistry suggests. The difference here is not extreme, but it can be much more. Glaze chemistry is relative, not absolute. It works best when you are changing material amounts, not material types. When you do introduce a very different mineral then you have a different system which has its own relative chemistry.

Two stains. 4 colors. Will the guilty oxide please step forward.

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One stain produces clear in one glaze and red in another. Another stain blue and purple. Why?

We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company they need to know the chemistry of your glaze to help.

Ceramic Oxide Periodic Table

All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).

This is crazing. Crazing is bad on functional ware.

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A crazing glaze on a porcelain mug

The left glaze is "stretched" on the clay so it cracks (it calculates to a high thermal expansion so this is not a surprise). This usually appears after firing but can appear years later. When the lines are close together like this it is more serious. If the effect is intended, it is called "crackle" (but no one would intend this on functional ware). The one on the left calculates much lower - and stays uncrazed indefinitely. Potters, hobbyists and artists invariably bump into this issue whether using commercial glazes or making their own.

"Art language" solutions don't work, at least some technical words are needed to even understand what this simply is: A mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very small, ceramics are brittle so if a glaze is stretched on the ware, it will crack to relieve the stress. Crazing appears when ceramic is cooled and the glaze contracts more than the clay to which it is rigidly attached.

Growing incredible crystals in a ceramic glaze is about the chemistry

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Close up of glaze crystals

Closeup of a crystalline glaze. Crystals of this type can grow very large (centimeters) in size. These grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. The fluxes are dominated by K2O and Na2O (from frits) and the catalyst is zinc oxide (enough to contribute a lot of ZnO). Because Al2O3 stiffens glaze melts, preventing crystal growth, it can be almost zero in these glazes (clays and feldspars supply Al2O3, so these glazes have almost none). The firing cycles involve rapid descents, holds and slow cools (sometimes with rises between them). Each discontinuity in the cooling curve creates specific effects in the crystal growth. These kinds of glazes are within the reach of almost anyone today since electronic controller-equipped kilns are now commodity items and anyone can fiddle with the chemistry and manage the testing of glazes in their insight-live.com account.

Same high-iron glaze. One crystallizes and the other does not. Why?

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Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the right, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the left. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the exact temperature they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.

Example of a whole rock chemical analysis lab report

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Powdered samples were sent to the lab. The numbers shown on this report are in percentage-by-weight. That means, for example, that 15.21% of the weight of the dry powder of Alberta Slip is Al2O3. Insight-live knows material chemistries in this way (whereas desktop Insight needs them as formulas). Some non-oxide elements are quantified as parts-per-million (these amounts are not normally high enough to take into account for traditional ceramic purposes). The LOI column shows how much mechanically and chemically bound water are gassed off during firing of the sample. The total is not exactly 100 because of inherent error in the method and compounds not included in the report.

This cone 6 transparent looked good, but I still improved it alot

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The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at insight-live.com. In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt-flow GLFL test comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica. This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear. See the article to find out.

Comparing glaze melt fluidity balls with their chemistries

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Three glaze balls melting down into a pool

Ten-gram GBMF test balls of these three glazes were fired to cone 6 on porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as clean as the frit. Notice the calculated thermal expansions: The greater melting or #2 and #3 comes at a cost, their thermal expansions are considerably higher, so they will be more likely to craze. Which of these is the best for functional ware? #1, G2926B. Its high SiO2 and enough-but-not-too-much B2O3 make it more durable. And it runs less during firing. And does not craze.

Comparing two glazes having different mechanisms for their matteness

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These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio. It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.

The unexpected reason for this crazing can be seen in the chemistry

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Glaze chemistry explains the crazing

The glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? This is an excellent example of the value of looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the COE of the whole way up. And talc? It contains SiO2 (so the SiO2 is not driven down nearly as much) and its MgO has a much lower expansion than CaO does.

A pottery glaze is settled, running and crazing. What to do?

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Blue glaze with chemistry calculations

The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay. Adjustment 1 (column 3 in the picture) eliminates the feldspar and sources Al2O3 from kaolin and KNaO from Frit 3110 (preserving the glaze's chemistry). To make that happen the amounts of other materials had to be juggled. But the fired test revealed that this one, although very similar, is melting more (because the frit releases its oxides more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition. SiO2 is the glass former, the more a glaze will accept without losing the intended visual character, the better. The result is less running and more durability and resistance to leaching.

A large glaze batch mixing error rescued using glaze chemistry

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A screen-shot from an Insight-live.com account showing the original and adjusted recipes side-by-sid

The person used Frit 3134 instead of 3124, it makes up 70% of the recipe. The glaze melted much more and ran off the ware. That sounds like an impossible-to-fix problem. It just so happens that these frits have very similar chemistry except for one thing: 3134 has almost no Al2O3. That means that kaolin can be added to the bad batch to source the missing Al2O3 (and replenish the shortage of SiO2 at the same time). Extra silica is also needed to restore the full SiO2. The new chemistry is not an exact match, the B2O3 is a little higher, but this should not be an issue. Of course a raw:calcine mix of kaolin is needed (to prevent the glaze from shrinking too much on drying and therefore cracking). From this calculation we can see that for every 100 grams of the original powder we need to add 10 EPK, 15 calcined kaolin and 17 silica. Of course, one would need to know the water content of the slurry, that is calculated as (weight wet - dry weight)/wet weight * 100. If the slurry was 50% water, for example, then every 200 grams of slurry would contain 100 grams of powder.

Substituting a frit to source B2O3 leads to a dead end

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Insight-live screen

In the new recipe I am trying to match the oxides (the white, yellow, red, blue boxes). B2O3 is priority, I have supplied enough of the new frit to almost match it, but that far overshoots the Na2O (even if all of the feldspar is removed). While differences in the Al2O3 and SiO2 can be compensated by adjustments in the feldspar, clay and silica, the new frit has far more Na2O and only half the B2O3. The match shown here is the best I am able to achieve, its biggest problem is the significant rise in the calculated thermal expansion, that is going to mean issues with crazing. The only way to use this frit to better match the chemistry of the original glaze would be in combination with another one that has much higher B2O3 and has as little Na2O as possible.

What can you do using glaze chemistry? More than you think!

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And Insight-Live screenshot

There is a direct relationship between the way ceramic glazes fire and their chemistry. These green panels in my Insight-live account compare two glaze recipes: A glossy and matte. Grasping their simple chemistry mechanisms is a first step to getting control of your glazes. To fixing problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. To substituting frits or incorporating available, better or cheaper materials while maintaining the same chemistry. To adjusting melting temperature, gloss, surface character, color. And identifying weaknesses in glazes to avoid problems. And to creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. And even to creating glazes from scratch and using your own native materials in the highest possible percentage.

How to include stains in chemistry calculations in Insight

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The simple answer is that you should not. The chemistry of stains is proprietary. Stain particles do not dissolve into the glaze melt like other materials, they suspend in the transparent glass to color it. That is why stains are color stable and dependable. In addition, their percentage in the recipe, not the formula, is the predictor of their effect on the fired glaze. Of course they do impose effects on the thermal expansion, melt fluidity, etc., but these must be rationalized by experience and testing. But you can still enter stains into Insight recipes. Consider adding the stains you use to your private materials database (for costing purposes for example).

Cutlery marking here is directly related to the chemistry of the glaze

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This is an example of cutlery marking in a cone 10 silky matte glaze lacking Al2O3, SiO2 and having too much MgO. Al2O3-deficient glazes often have high melt fluidity and run during firing, this freezes to a glass that lacks durability and hardness. But sufficient MgO levels can stabilize the melt and produce a glaze that appears stable but is not. Glazes need sufficient Al2O3 (and SiO2) to develop hardness and durability. Only after viewing the chemistry of this glaze did the cause for the marking become evident. This is an excellent demonstration of how imbalance in chemistry has real consequences. It is certainly possible to make a dolomite matte high temperature glaze that will not do this (G2571A is an example, it has lower MgO and higher Al2O3 and produces the same pleasant matte surface).

These two frits have one difference in the chemistry: Al2O3.

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These two boron frits (Ferro 3124 left, 3134 right) have almost the same chemistry. But there is one difference: The one on the right has no Al2O3, the one on the left has 10%. Alumina plays an important role (as an oxide that builds the glass) in stiffening the melt, giving it body and lowering its thermal expansion, you can see that in the way these flow when melting at 1800F. The frit on the right is invaluable where the glaze needs clay to suspend it (because the clay can supply the Al2O3). The frit on the left is better when the glaze already has plenty of clay, so it supplies the Al2O3. Of course, you need to be able to do the chemistry to figure out how to substitute these for each other because it involves changing the silica and kaolin amounts in the recipe also.

What happens when ceramic glazes lack Al2O3?

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This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.

Glazes of the same chemistry: The fritted one melts better

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It seems logical (and convenient) to just say that the kiln does not care what materials source the oxides in a glaze melt. Li2O, CaO, Al2O3, SiO2 are oxides (there are about ten common ones). The kiln just melts everything and constructs the glaze from the ones available. Right? Wrong! Things get more complicated when frits are introduced. Frits are man-made glasses, they melt much more readily than raw materials like feldspar. Raw materials are often crystalline. Crystals put up a fuss when asked to melt, often holding on as long as they can and then suddenly melting. Frits soften over a range and they start melting early. To illustrate: These two glazes have the same chemistry. But the one on the left sources sodium and alumina (Na2O3, Al2O3) from the 48% feldspar present. The other sources these from a frit (only 30% is needed for the same amount of Na2O3). The remainder of the recipe has been juggled to match the other oxides. The frit version is crystallizing on cooling (further testament to how fluid the melt is). What has happened here is great. Why? First, the chemistry has not changed (fewer firing differences). The frit has no Al2O3, it is being sourced from kaolin instead, now the slurry does not settle like a rock. Even better, silica can be added until the melt flow matches (might be up to 20%). That will drop the thermal expansion and reduce crazing. The added SiO2 will add resistance leaching and add durability. Frits are great! But you need to know how to incorporate them into a recipe using a little glaze chemistry.

A Limitation of the Seger Unity Formula

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Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide a different system with its own relative chemistry is entered. An extreme example of this would be to source Al2O3 to a glaze using calcined alumina instead of kaolin. Although the formula may be exactly the same, the fired result would be completely different because very little of the alumina would dissolve into the glaze melt. At the opposite extreme, a different frit could be used to supply a set of oxides (while maintaining the overall chemistry of the glaze) and the fired result would be much more chemically predictable. Why? Because the readily and release their oxides the the melt.

Frits vs. raw materials in glazes: It is not just about the chemistry

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I have done alot of work recently comparing the sourcing of fluxing oxides from frits vs. raw materials. The differences are often incredible. The oxides MgO and CaO normally come from materials that melt high. But common frits that source them melt low. The chemistry in the two cone 6 glazes (comparing on this melt flow tester) are the same. But Y4 sources the KNaO from Ferro Frit 3110 instead of feldspar and alot of the MgO from Ferro Frit 3249 instead of talc. Y4 would flow even more but also has 10% calcined alumina! Sourcing Al2O3 from such a refractory material should make it melt less than sourcing it from kaolin, but the frits overcome even that to create this amazing fluidity in a matte glaze. The lesson: All glazes have a chemistry, but that cannot be taken in isolation from the materials that source it.

Frits work much better in glaze chemistry

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The same glaze with MgO sourced from a frit (left) and from talc (right). The glaze is 1215U. Notice how much more the fritted one melts, even though they have the same chemistry. Frits are predictable when using glaze chemistry, it is more absolute and less relative. Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide in a glaze a different system with its own relative chemistry is entered. But when changing form one frit to another to supply an oxide or set of oxides, the melting properties stay within the same system and are predictable.

Adding a frit of unknown chemistry in a glaze recipe?

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That is like adding a dog of unknown breed to this team. How predictable is that going to be? It is like that with ceramic glazes. They fire the way they do because of their chemistry. Not knowing the chemical makeup of a key ingredient robs you of the single biggest tool to explain characteristics or issues or propose adjustments, improvements or fixes.

Inbound Photo Links



How and where to have a glaze tested to learn its chemical analysis

Melt flows of a commercial glaze and my duplicate
How to reverse-engineer a commercial transparent glaze

Insight-live oxide sourcing
How to choose ceramic materials to source needed oxides


Sometimes it is better to replace the base in a production glaze recipe

Links

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URLs http://en.wikipedia.org/wiki/Ceramic_chemistry
Ceramic chemistry on Wikipedia
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URLs http://insight-live.com/insight/quotes.php?keyword=chemistry
Does anyone care about glaze chemistry?
URLs https://www.khanacademy.org/science/chemistry
Inorganic chemistry videos Khan Academy
Articles Where do I start in understanding glazes?
Break your addiction to online recipes that don't work or bottled expensive glazes. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and drying properties. Even some chemistry.
Articles Stoneware Casting Body Recipes
Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them
Articles Chemistry vs. Matrix Blending to Create Glazes from Native Materials
Is it better to do trial and error line and matrix blending of materials to formulate your glazes or is it better to use glaze chemistry?
Articles Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity
Part of changing your viewpoint of glazes, from a collection of materials to a collection of oxides, is learning what a formula and analysis are, how conversion between the two is done and how unity and LOI impact this.
Glossary Medium Temperature Glaze
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.
Glossary Frit
Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials.
Glossary Boron Blue
Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
Glossary Limit Formula
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
Glossary Ceramic Oxide
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Glossary Decomposition
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization
Glossary Triaxial Glaze Blending
In ceramics many technicians develop and adjust glazes by blending two, three or even four l materials or glazes together to obtain new effects
Glossary Glaze Mixing
In ceramics, glazes are developed and mixed as recipes of made-made and natural powdered materials. Many potters mix their own, you can to. There are many advantages.
Glossary Characterization
In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.
Glossary Unity Formula
The chemistry of ceramic glazes are normally expressed as formulas. A unity formula has been retotaled to make the numbers of flux oxides total one.
Glossary Ceramic Glaze
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Glossary Ceramic Glaze Defects
Projects Oxides
Oxides KNaO - Potassium/Sodium Oxides
Media How I Formulated a Cone 6 Silky Matte Glaze Using Insight-Live
I will show you how found a recipe on Facebook, assessed it, substituted my own materials, tested it, adjusted it. Now it is like a cone 10 dolomite matte.
Media Remove Gerstley Borate and Improve a Popular Cone 6 Clear Glaze
How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
Media Compare the Chemistry of Recipes Using Insight-Live
Watch me open three recipes side-by-side and turn calculation on for each (in this 19 second video).
Media Substituting Materials by Weight: Why it does not work!
Wollastonite is 50:50 CaO:SiO2. So why not just substitute 40 wollastonite for 20 calcium carbonate and 20 silica? The answer will help you see reason why we make such a big deal of glaze chemistry.
By Tony Hansen
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